When were the accounts opened?
If it was before October 1, 2003 then the CIP regulation was not in effect. There was no legal requirement that the bank obtain the four pieces of information, including an identifying number. As they are demand deposit accounts, there was not even a need on the bank's part to obtain a W8-BEN. Label any effort you undertake as "due diligence" not CIP, but it probably would be prudent to clean this up.
If it was after the above date, review the bank's CIP and see how far out of compliance they were in opening these accounts. Regardless of what you find there, start contacting the account holders and give them a specific period of time (30 days?) to provide the information and documentation or the accounts will be closed. Document your efforts and finish the project in the timeframe you establish.